This invention relates to an apparatus and method for detecting an overstroke, understroke and/or underspeed condition for a piece of vibrating equipment and also to an apparatus which may be attached to vibratory equipment.
The present invention is particularly suited for use with vibratory equipment, such as for example, vibrating screens and conveyors which operate for long periods of time and have amplitudes or frequencies of vibrations which need to be monitored but heretofore have not been monitored. Such vibrating equipment may be part of a production line where a continuous stream of material travels along a vibratory conveyor and/or across a vibratory screening classifier which separates the material thereon by size and/or density. Typically, such equipment undergoes vibratory motion in a direction which is generally parallel to the direction the stream of material is intended to travel. If one part of the vibrating equipment malfunctions, as by vibrating at too great or too small an amplitude, the entire production line may be affected. For example, if one piece of vibrating equipment on a conveyor line vibrates too slowly, less material carried on the conveyor will pass downstream, thus creating a backup of material upstream of the malfunctioning equipment. Conversely, if a piece of equipment vibrates at too great an amplitude or frequency, the equipment can break down, thus requiring an entire production line to be shut down in order to fix the malfunctioning equipment. Often such equipment is unattended and may be in remote areas. Heretofore, a considerable time lag has occurred between the initiation of the equipment's malfunction and the discovery of the malfunction and, in addition, another time lag occurs between the discovery of the malfunction and the shutting down of the production line.
During these time lags, material on the production line can overflow the conveyor, or backup along the entire conveyor line. Often the material on the equipment must be hand shoveled or otherwise manually displaced from the equipment and this is also an expensive and time-consuming operation. Also, the equipment itself may break if allowed to vibrate too rapidly.
In order to detect the malfunctioning of high speed vibratory equipment, it is desired to immediately detect when the equipment is vibrating at an amplitude and/or frequency which is outside of a predetermined range which is compatible with the operating requirements of the equipment. A major problem with monitoring such vibratory equipment is that the equipment operates billions of cycles during its life and the monitoring apparatus must also be capable of operating for billions of cycles. Typically, the cycles per minute are low, for example, about 300 to 1800 cycles per minute and the amplitude of displacement is small, for example, usually about 0.125 to 1.50 inch. But the equipment often is not shut down and runs twenty-four hours per day and 365 days a year. Thus, such equipment soon reaches over a billion cycles per year; and the equipment is expected to operate for eight, ten or more years. A further factor is cost, particularly on retrofitting existing equipment in the field. If the cost of the monitoring apparatus is high, then the conventional manner of operating such equipment without monitoring will be maintained. The monitoring equipment also should be capable of being exposed to dirt, dust and other hostile environmental conditions.
The prior art does include vibration detectors, which operate as inertia detection switches, to provide an output signal when the amplitude of vibration of a piece of vibrating equipment exceeds a predetermined value. One such inertia switch is disclosed in U.S. Pat. No. 3,459,911 and it comprises a spring-biased magnet or weight carried in a housing. The magnet normally occupies a "rest" position. When the amplitude of vibration becomes too large, the magnet or weight is displaced from the rest position to a second position which activates a switch and which can be used to either sound an alarm to alert an operator that the amplitude vibration is too large, or to activate a control circuit to shut down the machine. The problem with the inertia switch is that it can only detect when the amplitude of vibration of the vibrating equipment is too large (overstroke).
Another device which might possibly be used to detect the amplitude and frequency of vibration of vibratory equipment is a strain gauge, a thin piece of metal which produces an output voltage as it is deformed. However, strain gauges, as disclosed in U.S. Pat. No. 3,201,776 are not thought to be suitable for continually monitoring vibration and vibratory equipment having a life operation over 10,000,000 cycles which is the typical life of a conventional strain gauge.
Thus, there is a need for an apparatus capable of monitoring vibratory equipment over a large number of cycles which is durable, and which can provide output signals to either warn an operator of overstrokes or understrokes, or to provide overstroke and understroke signals to control circuits to trigger an adjustment of the equipment to prevent breakdown of the equipment and also to prevent problems from occurring in the entire production line.